Bohr's way to defining complementarity(*) Alberto De Gregorio via del Colle di Mezzo, 21 – 00143 Rome, Italy (tel. no. +39 (0)6-5042895) e-mail: [email protected] Abstract. We go through Bohr’s talk about complementary features of quantum theory at the Volta Conference in September 1927, by collating a manuscript that Bohr wrote in Como with the unpub- lished stenographic report of his talk. We conclude – also with the help of some unpublished letters – that Bohr gave a very concise speech in September. The formulation of his ideas became fully de- veloped only between the fifth Solvay Conference, in Brussels in October, and early 1928. The un- published stenographic reports of the Solvay Conference suggest that we reconsider the role that discussions with his colleagues possibly had on Bohr’s final presentation of the complementary sides of atomic physics in his 1928 papers. Keywords Bohr Complementarity Volta Conference Solvay Conference There casually happened (as was usuall) several discourses at times between these Gentlemen, the which had rather inflamed than satisfied in their wits the thirst they had to be learning; whereupon they took a discreet resolution to meet together for certain dayes, in which all other business set aside, they might betake themselves more methodically to contemplate the Wonders of God in Heaven, and in the Earth. Galileo Galilei (Dialogues on two World Systems: to the Judicious Reader, tr. Th. Salusbury) Besides, there is no scientific work that one man alone can write. Bertolt Brecht (Life of Galileo: Scene XIV) 1. Introduction As Galilei and Brecht vividly remind us, science is intrinsically relational – like any other human activity. The right enthusiasm and admiration, we may feel before any milestone achievement in physics and the person who eventually reached it, do not allow us to underestimate the more or less thick, more or less evident web of human relationships and professional interactions which allowed that result to take shape. As a matter of fact, sometimes the personality of the discoverer may instead (*) This paper is based on, and further develops, Bohr’s talk at the 1927 Como Conference, a talk the author gave at the Second International Conference on the History of Quantum Physics in Utrecht in July 2008, and on which De Gregorio & Sebastiani (2009) is also based. 1 overshadow the whole context from which his discovery arose. Niels Bohr gave his first public accounts of complementarity in the second half of 1927. He had two main occasions: the Conference for the centenary of the death of Alessandro Volta, in Como in September, and the Fifth Solvay Conference, in Brussels in October. A wide and detailed account of the genesis of the idea of complementary features of quantum physics is in the sixth volume of Niels Bohr collected works, edited by Jørgen Kalckar and endowed with numerous letters and handwritten documents. Sometimes Kalckar explicitly states there, and more often implicitly assumes, that already in his speech at the Volta Conference in Como did Bohr thoroughly expound on complementarity. Cassidy, Pais, and Mehra and Rechenberg, seem to conform to Kalckar’s assumption as well. However, no document is provided, showing that Bohr had fully developed his idea of complementary aspects of atomic theory at the time of the Volta Conference, nor that his speech in Como was exhaustive to any extent. In this paper, we shall review Bohr's early account of the complementary features of the description of nature. Based on documental evidence, we will move along two lines. Our first course will be that Bohr's presentation of his ideas was still at an early stage of gestation in Como. We shall collate two documents: a draft titled Fundamental problems of the Quantum theory, sketching out some essentials of complementarity and written in Como on September 13 by Bohr, and the stenographic report of the speech that Bohr gave at that Conference on September 16. On the basis of this comparison, and with the help of some unpublished letters – kept at the Niels Bohr Archive – between the Committee of the Conference and Bohr, we shall conclude that the latter’s speech in Como in September was very concise. It contained only embryonic ideas about complementary features of quantum theory. In Brussels in October Bohr made in public an improved exposition of his views on the complementary sides of atomic physics, which he further substantiated in the detailed accounts he would publish in the Proceedings of the Volta Conference, in the Proceedings of the Solvay Conference, on Naturwissenschaften, and on Nature, in 1928. As for our second course, the role of Bohr's interaction with his colleagues will be reviewed in the light of the meagre content of Bohr's speech at Como. We shall start from a manuscript of October 12-13, 1927, which was discussed with Darwin and Pauli by Bohr in Italy, after the Volta Conference, and which improved the unsubstantial manuscript of September 13: we shall consider that these discussions allowed a substantial improvement of Bohr's presentation of complementary sides of nature, rather than a mere refinement of an already developed presentation. Bohr further improved the formulation of his views on the eve of the Solvay Meeting, or maybe in the very days of the Meeting. In Brussels, he discussed topics he had not included in his October manuscript (for example the gamma-ray microscope); he apparently treasured also discussions with his colleagues: 2 some issues examined with his peers (for instance the Stern-Gerlach experiment or the scattering of alpha-particles) are to be found in his final presentation. 2. The Volta Conference "Dear Colleague, next year the town of Como will celebrate the centenary of the death of […] Alessandro Volta. […] We pray you that you would honour the Congress with your presence […]. We would very much appreciate it if you would agree to communicate an original work of yours at the Conference."1 With a letter from Milan, dated July 5, 1926, the Committee for the Celebrations in honour of Volta invited Bohr to take part in the Como Conference of September 1927. Bohr was expressly asked, more than one year in advance, to deliver a speech during the Congress. Bohr did not answer this letter (yet he answered H.A. Lorentz's invitation, of June 7, 1926, to attend "another 'Conseil de Physique' to be held in Brussels [...] in October 1927"2). On June 14, 1927, almost one year after the invitation to Como, the President of the Committee sent Bohr another letter, from Bologna: "Dear Colleague, your Academy has just communicated to us that you and Dr. Kramers will come on their behalf to the Como Conference. […] We sent an invitation letter to you last July, one of the first being sent […]. I would regret very much if you had not received that letter. […] You can find the updated list of the contributions here attached. […] They will be published in a volume, and of course can be longer than the oral speeches."3 Obviously, Bohr had received the letter of July 5, 1926, now kept at the Bohr Archive. Even if the Committee had not received any answer from Bohr, by June 1927 he had already accepted to take part in the Conference on behalf of the Danish Academy. Obviously as well, the participants, Bohr included, were expected to deliver speeches shorter than the subsequent printed papers. On June 20, 1927, Bohr was ready to communicate the title of his contribution: Fundamental problems of the Quantum theory.4 The same title would be given to the already mentioned manuscript written in Como on September 13,5 but that title would be changed into The quantum postulate and the recent development of atomic theory in the Proceedings of the Conference.6 Bohr 1 Letter to Bohr, dated Milan 7/5/1926 (Niels Bohr Archive, Folder VOL27). The letter was signed by Quirino Majorana (President of the Committee for Scientific Conferences), Enrico Musa (General Clerk of the Executive Committee), and Enrico Médail (President of the Executive Committee). The letters between Bohr and the Committee are in French (author's translation). 2 Bohr answered to Lorentz on 6/24/1926. See Mehra & Rechenberg (2000, p. 175). 3 Letter of Q. Majorana to Bohr, dated Bologna 6/14/1927 (Niels Bohr Archive, Folder VOL27). 4 Rough copy of Bohr's letter to Q. Majorana, dated Copenhagen 6/20/1927 (Niels Bohr Archive, Folder VOL27). 5 Kalckar (1985, pp. 75-80). 6 Bohr (1928a). 3 also asks Q. Majorana information about the suitable length of his speech and about its publication. On July 4, 1927, the Committee sent Bohr another letter, providing us with further, precious information: «Dear Colleague, […] given the high number of planned speeches, it would be fine to limit them to twenty minutes each. […] We would be very grateful to you, if you sent us in advance a short summary of your talk. You would also facilitate the duties of the Committee if, before the Congress starts, you sent us the definitive version of the paper to be published. It would help make the composition faster».7 Three pieces of information are worth noting: as early as July 4, 1927, Bohr was asked i) to send a short summary in advance, and ii) to provide a complete paper before the Congress started; iii) the talks were expected to be twenty minutes long each. On August 24, Bohr answered that he regretted that he had been ill and could not finish the paper he was asked for. However, the Abstract of his talk was ready, and he attached it: «Fundamental problems of the quantum theory.